This invention relates to a heat treatment process for steel wire rods, and, more particularly, to a process for directly heat treating steel wire rods by utilizing the heat produced during the finish rolling of steel wire rods.
Heat treatment is necessary to impart high strength and toughness to hot-rolled hard-steel wire rods. Lead patenting is a common heat treatment process conventionally employed in the production of high-strength rods. In addition to this, simpler direct heat treatment processes utilizing the sensible heat (800.degree. to 1000.degree. C.) conserved in hot-rolled rods have been developed. For example, Japanese Provisional Patent Publications Nos. 38426 of 1981 and 102524 of 1981 proposed processes to dip as-rolled rods directly in a solution of salt.
A process disclosed in Japanese Provisional Patent Publication No. 38426 of 1981 uses a low-temperature bath of molten salt 14 and a high-temperature bath of molten salt 15, as shown in FIG. 3. A coil of steel wire rod 1 falling onto a roller conveyor 3 from a laying head 2 of a take-up reel moves forward in an unconcentric spiral. The moving rod is first cooled in the low-temperature bath of molten salt 14 in which a sorbite structure is formed and then in the high-temperature bath of molten salt 15 where untransformed austenite is completely transformed into sorbite. This process requires both a low-temperature bath for quenching and a high-temperature bath for retention, as one bath cannot provide adequate cooling.
Another process disclosed in Japanese Provisional Patent Publication No. 102524 of 1981 obtains a fine structure of pearlite by uniformly cooling wire rods in a bath of molten salt whose rate of heat transfer is increased by agitating with air or other gases satisfying specific requirements.
Though the processes employing two or one bath of molten salt just described produce wire rods having high strength and toughness comparable to those obtained by lead patenting, they involve the following new problems.
In the former process, undercooling of the surface and subsurface area to a temperature considerably lower than that desirable for transformation (which is substantially equal to the temperature of the high-temperature bath for retention) produces bainite. The use of two baths, one for quenching (at approximately 400.degree. C.) and one for retention (at approximately 550.degree. C.), necessitates troublesome temperature control of the individual baths as well as higher equipment investment and running cost. The latter process also gives rise to a problem of bainite formation resulting from the undercooling of the surface.